In the recent years, biometrics, and especially fingerprint sensors, have become common for the purpose of verifying the identity of a person, e.g., at immigration control, at airports as well as with personal devices such as laptops etc. The present solutions still have a number of disadvantages. Fingerprint sensors used in airports and immigration control are large and too expensive for many applications, smaller sensors seen in portable devices are often silicon based solutions with limited robustness and challenging electronic interconnections. Traditional silicon production techniques for such sensors often result in solutions for electrical interconnection features interfering with the physical finger interface of the device. Recessed mounting of the sensor in a consumer application is often implemented to improve these shortcomings, but may not be the optimal solution both with respect to esthetical design and protection from dirt and moisture. Sensor size, both volume and area, along with the rigid properties of silicon, significantly limits the feasibility of integrating fingerprint devices in thin and flexible applications such as smartcards.
A fingerprint sensor which may be flush mounted in the same plane as the surface of the product it is mounted into is described in U.S. Pat. No. 7,251,351, in which a set of first electrodes/sensor elements are positioned on one side of an insulating substrate provided with through-substrate-via conductors. The substrate may be made from glass, ceramics or other insulating materials. In WO 03/049012 a substrate made from a multilayered or laminate PCB process is described being based on a subtractive PCB process in which, as with the abovementioned US patent, is based on the removal of materials, e.g., by etching, which has relatively low resolution and thus not sufficiently good for the small dimensions and tolerances of fingerprint sensors. If the dimensions such as the layer thickness are not sufficiently accurate they may affect the measurements and reduce the accuracy of the sensor unit.
Thus it is an object of the present invention to offer a thin, flexible fingerprint sensor realized by well established, high volume, low cost manufacturing processes, while also allowing the sensor surface to be positioned flush with the surface of the device in which it is mounted. This is accomplished with a fingerprint sensor as stated above being characterized as described in the independent claims.